This invention relates to a cable for local area network (LAN) use, and, more particularly, to a cable having relatively low crosstalk at high bit rates over relatively long distances.
With the continuing development and use of data processing devices such as, for example, computers, and, more particularly, devices depending upon digital signals, there has developed a need for signal transmission media that provide substantially error free transmission of both analog and digital data signals at high rates. However, there are numerous factors which work against such transmission, chief among which is crosstalk. One commonly used cable configuration is a core arrangement of unshielded twisted pairs of wires contained within a cable jacket, but unless certain precautionary design modifications are included in the cable, crosstalk becomes a severe problem with increasing frequencies.
The unshielded twisted pair has always been used for telephone transmission in the balanced, or differential mode, and of late, its latent transmission capability in the unbalanced mode has come to be recognized. Especially noteworthy is the twisted pair""s capability of transmitting rugged quantized digital signals as compared to analog signals. In such transmission, for example, a four pair cable, generally two of the pairs transmit signals in one direction to a computer system, for example, and two of the pairs transmit signals in the other direction, e.g., from the computer system. In any such arrangement, near end crosstalk (NEXT) can cause a serious signal degradation, especially at high bit rates with digital signals. There have been numerous schemes proposed for reducing the deleterious effects of NEXT upon the signal being transmitted. Examples of such arrangements are shown in U.S. Pat. No. 4,697,051 of Beggs et al., U.S. Pat. No. 4,873,393 of Friesen et al., and U.S. Pat. No. 5,424,491 of walling et al. In the Friesen et al. patent, the disclosure of which is incorporated herein by reference, there is shown a cable having two or more twist pairs, in which the twist frequency of each pair is different than the twist frequency of the other pair or pairs such that the increments of the twist frequency spacing between adjacent pair are non-uniform and the twist length of each pair does not exceed the product of approximately forte (40) times the diameter of an insulated conductor of each pair. The short twist length and the gathering together of the conductor pairs effectively reduces pair meshing, thus maintaining a physical separation (spacing) among the pairs that tends to reduce interaction therebetween, further reducing crosstalk. The cabling scheme also eliminated the need for shielding of the conductors.
The foregoing patents all disclose inventions directed to the reduction of NEXT, and at least part of the ability of these arrangements to reduce near end crosstalk stems from the two way, or bidirectional signal transmission which, in effect, through the electromagnetic interaction, produces at least some cancellation of NEXT.
There are systems or networks, however, in which large numbers of signal receiving and transmitting components are involved to the extent that, especially at high speeds, NEXT becomes limiting. In such cases, it is the practice to use two or more cable groups, with all of the pairs in one cable transmitting unidirectionally toward the stations or equipment of the network, and the pairs of the other cable transmitting from the stations. In this way, crosstalk isolation is achieved. With such unidirectional transmission, another form of crosstalk, commonly called far end crosstalk (FEXT) becomes a factor in the ability of the cable to deliver signals that have not been seriously or hopelessly degraded. Heretofore the cables that have been designed to reduce NEXT have demonstrated sporadic FEXT results, especially in Category 6 type cables, i.e., cables for 100 megahertz or higher signal transmission, and, in many cases, fail the test of reduced signal degradation. When it is appreciated that far end crosstalk in a unidirectional transmission arrangement is the summation of all the couplings between the twisted pairs, as opposed to the average coupling in a bidirectional system, it becomes clear that FEXT is an important factor to be considered.
The present invention is a twisted pair cable which is designed to produce FEXT performance that is consistently within acceptable limits, thereby insuring transmission of high speed (or high frequency) digital data, and is based upon the discovery that different twisted pairs within the cable produce such results where some of the pairs have opposing twists. In other words, some of the pairs have a right hand twist and some have a left hand twist. Conductor pairs with the appropriate opposed twists do not tend to physically mesh, hence, the effective transverse spacing between pairs is increased and substantially constant, thereby reducing the crosstalk, or coupling, between pairs. When these couplings are reduced, their summation is also reduced, hence FEXT is reduced.
In, for example, a cable having a core that has four pairs, A, B, C, D, there are six possible couplings, AB, AC, AD, BC, BD, and CD when the twists are all in the same direction, either right hand or left hand as in present day cables. On the other hand, if pair A is given an opposite direction twist from the other three, there are only three possible same direction couplings, BC, BD, and CD. If pairs A and B are given, for example, left hand twists and pairs C and D have right hand twists, there are only two possible same direction couplings, AB and CD. It can be appreciated that a decrease in the number of couplings is desirable in reducing FEXT.
Although the principles of the invention obtain with different numbers of twisted pairs within the cable, these principles are best illustrated with reference to four twisted pairs. Hence, in one embodiment of the invention, the cable thereof comprises four pairs of insulated wires enclosed in a tubular member of suitable plastic material. A metallic shield may or may not enclose the tubular member, and a plastic jacket preferably surrounds the assembly.
In accordance with the principles of the invention, a first twisted pair (pair A) has a left hand twist of, for example, approximately 0.380xe2x80x3 to 0.523xe2x80x3 twist length and a second twisted pair (pair B) has a left hand twist of, for example, approximately 0.390xe2x80x3 to 0.487xe2x80x3 twist length. A third twisted pair (pair C) has a right hand twist of, for example, approximately 0.580xe2x80x3 to 0.619xe2x80x3 twist length and the fourth twisted pair (pair D) has a right hand twist of, for example, 0.650xe2x80x3 to 0.713xe2x80x3 twist length. In a preferred embodiment, pairs A, B, C, D have values of approximately 0440xe2x80x3, 0.410xe2x80x3, 0.596xe2x80x3, and 0.670xe2x80x3, respectively. Thus, all of the twist lengths are both short and different, and separation among pairs is maintained, thereby reducing the meshing which produces a consequent enhanced coupling. It has been found that such a cable arrangement does not materially affect NEXT performance, although it does not, apparently, improve it, but that it does result in improved FEXT performance.
The cable of the invention also produces a physical as well as an electrical benefit. In present day twisted pair cables, wherein all of the twisted pairs have the same twist direction, i.e., left hand or right hand, a torsion is imparted to the cable such that in relatively long lengths of cable, the cable itself becomes difficult to manipulate during installation. The cable of the invention, on the other hand, is more mechanically or physically neutral inasmuch as there is less creation of torsion with the different pairs having different directions of twist.
The numerous principles and advantages of the present invention will be more readily apparent from the following detailed description, read in conjunction with the accompanying drawings.